1. Field of the Invention
This invention relates broadly to the field of sensors which are used to sense the position of an object, such as a positionable control element in an automated industrial process plant. More specifically, this invention relates to a positional feedback type servo mechanism for controlling a positionable element which is more precise, more durable, more adaptable to specialized applications and less affected by environmental factors than positioners which have commonly been used in the past.
2. Description of the Prior Art
Servo mechanisms for controlling various system components such as valves, variable pitch fans, variable stroke pumps and other components are in common use throughout industry, both in the United States and throughout the world.
In such mechanisms, a feedback signal proportional to the position of the control element of the system component is detected, and power is directed to an actuator to bring the actuator and, thus the control element, to a desired position. In an electrical servo mechanism, the feedback signal which represents the position of the final control element is most commonly derived from the position of a slide wire on a potentiometer. Most often, the servo mechanism includes a comparator which compares the feedback signal to an input signal which is representative of the desired position of the final control element. This input signal can be generated by a manually adjustable component, or by a central control system, the latter arrangement becoming more common as industry progresses toward automation. The comparator generates an error signal which is proportional to the magnitude of the difference between the feedback signal and the input signal. This error signal is amplified, and is ultimately utilized to direct the actuator to bring the final control element to the position which is specified by the input signal.
In the past, industrial process control systems have often relied on pneumatically signalled and controlled servo mechanisms to control process variables within the system. As time passes, however, pneumatically signalled positioners are being replaced by electrically signalled positioners in ever increasing numbers because of the higher signal transmission speed and compatibility to centralized computers. Electrically signalled positioners through use of the greater accuracy of digital signal processing, can achieve a higher degree of process accuracy.
Even as the use of electrically signalled positioners increases, it is becoming increasingly apparent that there are inherent limitations in the use of a potentiometer to generate the position feedback signal. As a result of the physical nature of its construction, the resistance of a potentiometer is not continuously variable throughout its range, which limits the sensitivity of a potentiometer based positioner. In addition, the wiper element of a potentiometer tends to wear after continued use, affecting the resistance characteristics of the potentiometer and thus the accuracy of the positioner. The motion of the wiper on the resistive element of the potentiometer can also generate noise in the positioning circuit, which can be detrimental to the circuit and to the control system in general. Furthermore, the wiper motion and thus the signal generated in relation to the actual position is usually linear and is difficult to nonlinearly characterize.
It is clear that there has existed a long and unfilled need in the prior art for an electrically signaled positioner which can be used in an servo mechanism for controlling the position of an element which is more precise, more resistant to wear, more resistant to interference from environmental factors, and which can be characterized to represent a particular positional control profile for an element which is being controlled.